Our previous studies have investigated the self-sustained multicentennial oscillation of the Atlantic meridional overturning circulation (AMOC) in a one-hemisphere box model. In this paper, we extend the one-hemisphere model to a two-hemisphere model, and study the roles of both the thermohaline and wind-driven circulations in the multicentennial oscillation. A similar self-sustained multicentennial oscillation of the AMOC in the two-hemisphere box model is identified. Several new findings are summarized as follows. First, the salinity advection feedback in the North Atlantic still plays the most important role in controlling the multicentennial oscillation of the AMOC, while that in the South Atlantic is unimportant. Second, in comparison to that in the one-hemisphere box model, the self-sustained multicentennial oscillation in the two-hemisphere box model exhibits higher probability of occurrence and less sensitivity to changes in basin geometry. Third, the wind-driven circulation, specifically, the subtropical cells, can weaken the oscillation amplitude because the negative feedback between the wind-driven and thermohaline circulations restrains the salinity advection feedback, while its effect on the oscillation period is negligible. Fourth, without the AMOC, there will be no multicentennial oscillation, suggesting that the AMOC serves as a sufficient and necessary condition for the multicentennial oscillation. Similar to previous studies, stochastic freshwater forcing can excite a sustained multicentennial oscillation, demonstrating that the multicentennial mode is an intrinsic feature of the AMOC. We also identified a damped millennial oscillatory mode in the two-hemisphere box model, which warrants further investigation in future research, as it may provide clues to understand the Dansgaard-Oeschger events that occurred in past climate.

Atlantic Meridional Overturning Circulation, box model, self-sustained multicentennial oscillation, thermohaline circulation, wind-driven circulation